http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53034
--- Comment #1 from Steven Bosscher <steven at gcc dot gnu.org> 2012-04-18 20:25:45 UTC --- The gimple switch conversion pass is much too aggressive, worse code is generated for the examples that were used to introduce the implementation of switch statements with bit tests. (see http://gcc.gnu.org/ml/gcc-patches/2003-01/msg01733.html) For example, "GCC: (GNU) 4.5.0 20091228 (experimental) [trunk revision 155486]" applied to the first test case: $ cat t.c int foo(int x) { switch (x) { case 4: case 6: case 9: case 11: return 30; } return 31; } $ gcc-4.5.0 -S -Os -fdump-tree-optimized -fdump-rtl-expand-slim t.c -m32 -fno-tree-switch-conversion && cat t.s .file "t.c" .text .globl foo .type foo, @function foo: pushl %ebp movl %esp, %ebp movl 8(%ebp), %ecx cmpl $11, %ecx ja .L4 movl $1, %edx movl $30, %eax sall %cl, %edx andl $2640, %edx jne .L2 .L4: movl $31, %eax .L2: popl %ebp ret .size foo, .-foo .ident "GCC: (GNU) 4.5.0 20091228 (experimental) [trunk revision 155486]" .section .note.GNU-stack,"",@progbits $ gcc-4.5.0 -S -Os -fdump-tree-optimized -fdump-rtl-expand-slim t.c -m32 && cat t.s .file "t.c" .text .globl foo .type foo, @function foo: pushl %ebp movl $31, %eax movl %esp, %ebp movl 8(%ebp), %edx subl $4, %edx cmpl $7, %edx ja .L2 movl CSWTCH.1(,%edx,4), %eax .L2: popl %ebp ret .size foo, .-foo .section .rodata .align 4 .type CSWTCH.1, @object .size CSWTCH.1, 32 CSWTCH.1: .long 30 .long 31 .long 30 .long 31 .long 31 .long 30 .long 31 .long 30 .ident "GCC: (GNU) 4.5.0 20091228 (experimental) [trunk revision 155486]" .section .note.GNU-stack,"",@progbits Without switch conversion, there are 12 instructions including 2 forward branches and no loads from memory and no rodata. With switch conversion there are 10 insns including one load and one branch. So size takes a penalty, and for throughput the load may be more expensive than the branch (for larger test cases anyway). $ gcc-4.5.0 -c -Os -fdump-tree-optimized -fdump-rtl-expand-slim t.c -fno-tree-switch-conversion -m32 && size t.o text data bss dec hex filename 38 0 0 38 26 t.o $ gcc-4.5.0 -c -Os -fdump-tree-optimized -fdump-rtl-expand-slim t.c -ftree-switch-conversion -m32 && size t.o text data bss dec hex filename 60 0 0 60 3c t.o For the second test case: $ cat t.c int bar(int x) { switch (x) { case '0': case '1': case '2': case '3': case '4': case '5': case '6': case '7': case '8': case '9': case 'A': case 'B': case 'C': case 'D': case 'E': case 'F': return 1; } return 0; } $ gcc-4.5.0 -c -Os -fdump-tree-optimized -fdump-rtl-expand-slim t.c -ftree-switch-conversion -m32 && size t.o text data bss dec hex filename 117 0 0 117 75 t.o $ gcc-4.5.0 -c -Os -fdump-tree-optimized -fdump-rtl-expand-slim t.c -fno-tree-switch-conversion -m32 && size t.o text data bss dec hex filename 38 0 0 38 26 t.o $ gcc-4.5.0 -c -Os -fdump-tree-optimized -fdump-rtl-expand-slim t.c -ftree-switch-conversion -m64 && size t.o text data bss dec hex filename 161 0 0 161 a1 t.o $ gcc-4.5.0 -c -Os -fdump-tree-optimized -fdump-rtl-expand-slim t.c -fno-tree-switch-conversion -m64 && size t.o text data bss dec hex filename 80 0 0 80 50 t.o Without switch conversion (-m64): .file "t.c" .text .globl bar .type bar, @function bar: .LFB0: .cfi_startproc leal -48(%rdi), %ecx cmpl $22, %ecx ja .L4 movl $1, %edx movl $1, %eax salq %cl, %rdx testl $8258559, %edx jne .L2 .L4: xorl %eax, %eax .L2: ret .cfi_endproc .LFE0: .size bar, .-bar .ident "GCC: (GNU) 4.5.0 20091228 (experimental) [trunk revision 155486]" .section .note.GNU-stack,"",@progbits With switch conversion enabled: .file "t.c" .text .globl bar .type bar, @function bar: .LFB0: .cfi_startproc subl $48, %edi xorl %eax, %eax cmpl $22, %edi ja .L2 movslq %edi, %rdi movl CSWTCH.1(,%rdi,4), %eax .L2: ret .cfi_endproc .LFE0: .size bar, .-bar .section .rodata .align 16 .type CSWTCH.1, @object .size CSWTCH.1, 92 CSWTCH.1: .long 1 .long 1 .long 1 .long 1 .long 1 .long 1 .long 1 .long 1 .long 1 .long 1 .long 0 .long 0 .long 0 .long 0 .long 0 .long 0 .long 0 .long 1 .long 1 .long 1 .long 1 .long 1 .long 1 .ident "GCC: (GNU) 4.5.0 20091228 (experimental) [trunk revision 155486]" .section .note.GNU-stack,"",@progbits
